1. Field of the Invention
The present invention relates to an apparatus for controlling a hydraulically operated continuously variable transmission for a motor vehicle. More particularly, the invention is concerned with an improvement in the hydraulic control arrangement for increasing the power transmitting capacity of such a continuously variable transmission with an increase in the input torque of the transmission.
2. Discussion of the Prior Art
In the art of transmitting power from the engine of a motor vehicle to the drive wheels, there is known a hydraulically operated continuously variable transmission whose speed ratio is steplessly or continuously variable and which is disposed in a power transmitting path between the engine and the drive wheels. Examples of such a continuously variable transmission (hereinafter referred to as "CVT", when appropriate) include a belt-and-pulley type CVT and a traction type CVT, wherein the power transmitting capacity is variable or adjustable.
The belt-and-pulley type CVT has a pair of variable-diameter pulleys, and a transmission belt connecting the pulleys to transmit power between the pulleys. The speed ratio of this type of CVT is changed by changing the effective diameters of the pulleys. Further, the pressure of engagement of the transmission belt with the pulleys is changed with a controlled pressure applied to one of two hydraulic cylinders provided for the respective two pulleys. Namely, the power transmitting capacity of the CVT is changed or adjusted by suitably controlling the pressure applied to the cylinder assigned to produce a tension of the belt.
On the other hand, the traction type CVT has an input cone member and an output cone member which are coaxially disposed in mutually facing relation with each other, and a power transmitting member in the form of rollers which are interposed between the input and output cone members, for transmitting power between the cone members. Each of the rollers has an axis of rotation which is pivotable in a plane which includes the axis of rotation of the cone members. The speed ratio of the traction type CVT is changed by pivoting the axes of rotation of the rollers, and the power transmitting capacity is changed by controlling the pressure of engagement of the rollers with the cone members, that is, the controlled pressure applied to a hydraulic cylinder assigned to produce a contact force between the rollers and the cone members. An example of the traction type CVT is illustrated in Publication No. 63-29142 of examined Japanese Patent Application.
Thus, the power transmitting capacity of the belt-and-pulley type CVT and traction type CVT is changed with or determined by the pressure of engagement of the power transmitting member in the form of the belt of rollers.
In the continuously variable transmission (CVT) as described above, it is proposed to maintain the power transmitting capacity or the pressure of engagement of the power transmitting member at a comparatively high level to avoid slipping of the power transmitting member, while the input torque received by the CVT is relatively high, for instance, while the output torque of a torque converter is larger than the input torque of the same, where the CVT receives power from the torque converter. More specifically, the pressure of engagement of the power transmitting member is changed depending upon the degree of torque amplification of the torque converter during the torque converting period. In a hydraulic control apparatus for a belt-and-pulley type CVT as disclosed in laid-open Publication No. 57-161361 of unexamined Japanese Patent Application, for example, the line pressure applied to establish the tension of the transmission belt is increased with the reaction force of the stator of a torque converter, as well as with the angle of opening of the throttle valve of the vehicle engine and the speed reduction ratio of the CVT (Nin/Nout=1/speed ratio, where Nin: input shaft speed; Nout: output shaft speed; and speed ratio=Nout/Nin). That is, the line pressure is increased to establish an optimum tension of the transmission belt, not only according to an increase in the transmission torque of the CVT as a result of an increase in the amount of opening of the throttle valve, but also in response to a sudden temporary increase in the input torque of the CVT due to the torque amplification of the torque converter, so as to avoid slipping of the transmission belt.
In the known apparatus for controlling the continuously variable transmission as described above, the line pressure must be almost doubled, for example, increased from 20 kg/cm.sup.2 to 40 kg/cm.sup.2 while the input torque of the transmission is considerably increased due to the torque amplification by the torque converter from which the transmission receives the input torque. To permit this increase in the line pressure, the oil pump and pressure regulating valve of the hydraulic system must have a sufficiently high degree of operational accuracy, and the components of the relevant hydraulic cylinder, body of the valve and body of the oil pump must have sufficiently high strength or rigidity. Further, the hydraulic system must use sealing members which have a high degree of pressure tightness. Accordingly, the hydraulic system as a whole tends to be large-sized and expensive. Another disadvantage arises from the above-indicated line pressure increase, which results in an increased amount of power loss of the oil pump.